MIC74

Micrel

MIC74

12

August 1, 2000

Initializing the MIC74

The MIC74’s internal registers are reset to their default state

at power-on. The MIC74’s default state can be summarized

as follows:

• All I/O’s configured as inputs (DIR = 00

• Output configuration set to open-drain

(OUT_CFG = 00

• All outputs high/floating (DATA = FF

• Fan functions disabled (FAN_SPEED = 00

FAN bit of DEV_CFG = 0)

• All interrupts masked (IE bit of DEV_CFG = 0)

The result of this configuration is that all I/O pins will essen-

tially float unless driven by external circuitry. Any system

using the MIC74 will need to initialize the internal registers to

the state required for proper system operation. The recom-

mended order for initializing the MIC74’s registers is as

follows:

Write desired output

values to DATA

Set output configuration

in OUT_CFG

Set desired I/O's as

outputs by writing DIR

Set initial fan speed in

FAN_SPEED (if using)

Read STATUS

to clear it

Write DEV_CFG to

turn on fan (if using)

Initialize for

polling

Initialization

complete

Figure 9a. Initializing the MIC74 for Polled Operation

Write desired ouput

values to DATA

Set output configuration

in OUT_CFG

Set desired I/O's as

outputs by writing DIR

Set initial fan speed in

FAN_SPEED (if using)

Write INT_MASK to

enable interrupts

(if using)

Read STATUS to

clear it

Write DEV_CFG to

turn on interrupts

and fan (if using)

Initialize for

interrupts

Initialization

complete

Figure9b. Initializing the MIC74 for Interrupts

1. Write DATA

2. Write OUT_CFG

3. Write DIR

4. Write FAN_SPEED (if using fan mode)

5. Write INT_MASK (if using interrupts)

6. Read STATUS to clear it.

7. Write DEV_CFG to enable fan mode and/or

interrupts, if using

At the conclusion of step three, any I/O’s configured as

outputs in step two will be driven to the levels programmed

into the data register in step one. The order of step 1 through

step 3 is important to insure that spurious data does not

appear at the I/O’s during configuration. Following step 7,

programming the device configuration register, the MIC74

will begin generating interrupts if they are enabled, and the

fan will be started if FAN_SPEED contains a nonzero value.

The corresponding interrupt service routines (if any) must be

initialized and enabled prior to step seven. STATUS should

be cleared (step 6) in both polled and interrupt driven sys-

tems.